Recycling or
Disposal?
Hazardous Waste Combustion in Cement Kilns

IV.
Hazardous Waste Combustion in Cement Kilns

Portland
cement is produced by heating calcium (usually limestone)
silica and alumina (typically clay or shale) and iron
(steel mill scale or iron ore) in cement kilns to
temperatures of up to 2,700 degrees Fahrenheit. Under
this intense heat, the raw materials blend and form a
pebble-like substance called "clinker." After
the clinker is cooled it is ground-up with a small amount
of gypsum to produce cement.

Cement kilns are basically tilted, rotating cylinders
lined with heat-resistant bricks. They vary in size
depending on the particular type of cement-making process
employed, and can reach 750 feet in length and 25 feet in
diameter. The raw feed material mixture
("meal") is fed into the higher, elevated or
"cool" end of the kiln. As the kiln slowly
rotates, the raw meal tumbles down toward the hot lower,
or "flame" end, gradually altering physically
and chemically in the intense heat to form clinker.

Cement kilns operate in a counter-current
configuration. Combustion gases enter the kiln at the hot
lower end and flow upward, heating the raw materials
flowing in the opposite direction as they pass over, and
exit the kiln at the top end. The gases (which reach
temperatures several hundred degrees hotter than the raw
material) then pass through air pollution control devices
before entering the atmosphere. These devices are
typically either a fabric filter or electrostatic
precipitator, both of which function to remove the
particulate matter entrained in the gas stream before the
gasses are emitted into the atmosphere. This particulate
matter is referred to as cement kiln dust or CKD.

Five thermal zones exist in cement kilns. The raw
material initially passes through the drying and
pre-heating zone, where the temperature of the raw
material is raised to about 1,480 degrees. Here the free
and chemically bound water evaporates. Next, in the calcinating
zone, the limestone is chemically converted into lime
at temperatures of up to 2,192 degrees. In the last three
zones, the upper transition zone, the sintering
zone and the lower transition or cooling
zone (sometimes referred to collectively as the burning
zone) the raw material is heated to its maximum
temperature of approximately 2,700 degrees and chemically
alters under the withering heat to form clinker.

There are three basic types of cement manufacturing
processes, wet process, semi-dry process, and dry
process. In the wet process the raw material is blended
with water to produce a slurry which is pumped directly
into the cold end of the kiln. The slurrying process
helps homogenize the material. The wet process is the
oldest of the three processes, and is also the most
energy intensive, because the water must be evaporated
out of the slurry mixture.

The semi-dry or Lepol process involves mixing a
smaller amount of water with the raw material which is
then exposed to the exit gases from the kiln prior to
entering the kiln chamber.

In the more energy efficient dry kiln process, the raw
material enters the kiln in a dry powdered form. Three
types of kilns utilize the dry process. The preheater
kiln features a tower of heat-exchanging cyclones. The
raw material enters the pre-heater in a dry powdered form
where it is pre-heated by the hot exit gases from the
kiln prior to entering the kiln chamber. The
pre-calcinator kiln is identical to the pre-heater kiln
except that a separate combustion gas inlet at the base
of the preheater promotes further calcination of the
material before entering the kiln. The third type of kiln
is referred to as the long dry kiln and feeds dry raw
material directly into the upper end of the kiln.

B. Lightweight
Aggregate Kilns:

Lightweight aggregates are clays, shale, or slate
material which have varying characteristics linked to
their geological formation. These materials are combined
with cement to produce concrete products. It is estimated
that 80% of lightweight aggregate cement production
utilizes the rotary kiln method, which is very similar to
the technique used to produce Portland cement described
above. Lightweight aggregate kilns that burn hazardous
waste typically use hazardous waste as their sole fuel.

C. Use of hazardous
waste fuel in cement kilns:

As one can readily conclude from the above
description, the extraordinarily high temperatures
involved in producing cement require large amounts of
energy. Manufacturing one ton of cement requires an
average of 4.4 million Btu - roughly equal to 400 pounds
of coal. Cement kilns use coal, oil, petroleum coke,
natural gas, or hazardous waste fuel. Most cement kilns
burning hazardous waste use it to supplement - rather
than replace - conventional fuel.

Most cement kilns burning hazardous wastes are the
"wet" process type. The reasons hazardous waste
fuel is used mostly by wet-process kilns are rooted in
the economics of the cement industry. As noted above, the
wet process is the most energy intensive of the cement
making processes. Since fuel costs are such a significant
part of the cost of producing cement, wet-process
facilities are under great pressure to reduce fuel costs.
Critics of the use of hazardous waste fuel by the cement
industry argue that energy "savings" through
the use of hazardous waste fuel are illusory because the
waste is burned by less energy-efficient facilities to
begin with, and that the practice will render the U.S.
cement industry less competitive in the long run by
slowing the phase-out of older, less efficient
wet-process facilities.

Liquid hazardous wastes are combusted at the lower or
"hot" end of the kiln. Solid hazardous wastes
can enter the kiln at one of several locations, most
commonly in the calcinating zone, but also directly into
the pre-calcinator vessel or preheater inlet in
pre-calcinator or preheater kilns, or by projection
devices into the hot end of the kiln.

D. Nature of
Hazardous Waste Fuel:

The term "hazardous waste" as used in this
paper means a waste material which is classified and
regulated as a hazardous waste under the Federal Resource
Conservation and Recovery Act (RCRA). RCRA regulations
define hazardous waste as a solid waste which is either
listed as a hazardous waste under 40 CFR Part 261 Subpart
D, or exhibits any of the four characteristics of a
hazardous waste found in 40 CFR Part 261 Subpart C. These
characteristics are ignitability, corrosivity, reactivity
or toxicity (based on the Toxicity Characteristic
Leaching Procedure (TCLP)).

Hazardous wastes used by cement kilns include spent
and off-specification industrial solvents from paint and
coatings, auto and truck assembly, solvent reclamation,
ink and printing, cosmetics, toy, medical and electronic
industry operations. Paint thinners, waste oils and other
petrochemical byproducts are also burned.

Not all hazardous waste is directly suitable for use
as BIF fuel. Non-combustible waste and waste with little
or no energy value obviously cannot be burned as
generated. The waste-burning cement industry has
published a narrative identifying wastes which it is
claimed the industry avoids: highly corrosive, reactive
or chlorinated waste fuels which could damage their
equipment or facilities, or waste fuels with high
concentrations of metals which could affect the quality
of the cement product.

However, it would be a mistake to conclude that
non-combustible, corrosive, reactive or chlorinated
wastes are not received in cement and aggregate kilns.
Fuel blenders routinely mix such wastes with higher fuel
value waste, and even with petroleum products to provide
hazardous waste derived fuel (HWDF) to kiln operators.
The practice has grown in recent years because
higher-fuel value wastes are increasingly recycled or
disposed of at the manufacturing facility. Consequently,
it has become more profitable for kilns, through their
fuel blenders, to accept poor or non-existent fuel value
solid waste and blend it with clean liquid solvents or
virgin oil to produce a "milkshake," that is, a
pumpable mixture of liquids and solid waste which can be
used as fuel.

RCRA Land Disposal Restrictions (LDRs) require
destruction, removal, or immobilization of the hazardous
constituent of wastes to levels achievable by the Best
Demonstrated Technology (BDT) before land disposal is
permitted.

The LDRs prohibit using "dilution" as a
substitute for adequate treatment because dilution does
not result in the destruction of the waste. EPA has
recently interpreted the LDRs to prohibit combustion of
certain inorganic metal bearing waste on grounds that
combustion constitutes dilution. The list includes 8
heavy metals typically emitted by BIFs (arsenic, barium,
cadmium, chromium, lead, mercury, selenium, and silver)
and 44 metal contaminated wastes including various
sludges and wastewater from electroplating operations,
production of metal alloys, or smelting operations.

The interpretation does not apply to any of these
wastes if they contain hazardous organic constituents
exceeding treatment levels; are contained in organic,
debris like material; or have a heating value greater
than 5,000 Btu. The organic constituents or heating value
must be contained in the waste at the point of
generation. This is important because it forbids fuel
blending to add organics to inorganic metal-bearing
waste.